Conventionally, a digital optical transmission technology is actively studied and developed and the technology of an optical communication network ranging to a trunk line series, a metro line series, an access series and so on is rapidly developed and becomes a familiar technology with the spread of FTTH. Recently, it is further strongly required to achieve high-speed signal transmission between boards of digital devices or computers. The optical transmission and interconnection technology (optical wiring technology) is also progressively put into practice. Further, the technology of optical interconnections that replaces wiring interconnections between chips or in chips is developed and is vigorously studied, which is expected to overcome a problem of a wiring interconnection bottleneck of an integrated circuit. This is because the optical signal transmission is excellent in the transmission speed and interference between signals and the like in comparison with the electrical signal transmission. A reduction in power consumption and miniaturization of an optical circuit, an optical module or an optical device which is utilized for an optical transmission or an optical interconnection is required with the background of a rapid increase in the recent communication traffics and a rapid increase in the necessity of interconnections that make use of high speed of light from between boards into a board or from between chips into a chip.
In the field of optical interconnection technology, there is developed a study of silicon-photonics in which a matured process technology in a silicon LSI can be utilized. Accordingly, a waveguide having a low loss even in an abruptly bent portion that is extremely minute (for example, the cross section is 500 nanometer square or less) can be realized by making use of a difference in a high refractive index between silicon and silicon oxide or air. Thus, it is possible to achieve a reduction in power consumption and miniaturization of a transmission and reception system or module for optical communication, and it is also possible to realize an introduction and integration of an optical interconnection into a silicon LSI. As the device for the optical interconnection, a miniaturized and highly-efficient optical coupling-branching device for optical coupling and optical branching in an optical device or between optical devices including an optical waveguide becomes an extremely important development factor together with an active device for light transmission, light modulation, light reception.
Particularly, in the optical coupling device, as expected, a spot-size converter that can be easily formed with a high reduction ratio (one to two digits or more) and high efficiency becomes a key device for practical use. Conventionally, as a spot-size converter that performs optical coupling with high efficiency, a method for combining an inverted taper structure of a silicon fine line with a clad for optical confinement is widely used since formation and integration become relatively easy by utilizing the feature of silicon suitable for a miniaturization process as disclosed in JP-A 2009-36873 (KOKAI).
However, for coupling with high efficiency, it is necessary to form a long (several hundred μm) taper whose head width is several ten nm or less. In this structure, since miniaturization patterning by electron beam (EB) lithography is required, a problem that it is not suitable for mass production occurs. Further, the taper length becomes extremely long to perform heat-insulating mode conversion and integration with high density becomes difficult and an important problem occurs particularly when it is applied to an optical interconnection on a small LSI chip.
Further, on the other hand, an optical coupling-branching device that efficiently distributes light to a plurality of waveguides becomes a key device for a highly efficient operation of a device having optical branches in an internal portion such as an optical interferometer which is used in an optical modulator or the like. The optical coupling-branching device is also important for high efficiency of optical coupling and branching required for a parallel process and multiplexing of signal processes. To realize this high efficient operation, there are often used (1) continuous branch waveguides and (2) an optical coupling-branching device utilizing multi-mode interference.
As described above, there is known a convention optical coupling device that reduces or enlarges a light beam with a high reduction ratio or high enlargement ratio for an optical interconnection on an LSI chip and couples the light beam with high efficiency. The convention optical coupling device has a problem that practical use and mass-production are difficult, since a high-degree miniaturization processing technology that is not suitable for mass production is required and the device length becomes extremely long.
Particularly, when a circuit is formed on the LSI chip by means of an optical interconnection, the optical waveguide or waveguides are crossed on or upon the other waveguide or waveguides so that the intersection between optical waveguides inevitably occurs. Thus, it is required to propose a method for reducing a loss or crosstalk at the intersection. Some methods for solving this problem are proposed, for example, (1) a method for devising the shape of an intersecting portion to prevent the mode shape of propagation light in the intersecting portion from being overlapped between the waveguides as far as possible by directly intersecting them, (2) a method for propagating one portion in the waveguide in the intersecting portion and propagating the other portion outside the waveguide (clad layer), and (3) a method for making an interconnection to cause the intersection to occur only between different layers by multi-laying the optical interconnections and the like. Among the above proposals, the method for multi-laying the optical interconnections to directly avoid the intersection is understood as a unique method that can completely eliminate a loss or crosstalk in the intersecting portion.
However, in the multi-layered interconnection structure, a waveguide that connects different layers or an optical coupling device that connects waveguides becomes necessary. Among them, in order to form a waveguide that connects the layers, a high-degree 3-dimensional process and 3-dimensional interconnection technology for forming a 3-dimensionally bent waveguide are required and it is understood difficult to realize the same. In comparison with this, it is extremely easy to couple the optical waveguides arranged on different layers via an optical coupling device that controls the height of light propagation. As a method for realizing this, a method for processing the head portions of two waveguides arranged on different layers into inverted taper forms and setting them to face each other to switch the layer used for propagating light is proposed in JP-A 2008-261952 (KOKAI). However, since the method requires extremely long taper length (several hundred micron to one millimeter or more) to highly efficient coupling of the inverted tapers although it is high performance, it is understood difficult to be applied to an optical interconnection with high density on the chip.
Further, it is pointed out that the following problem occurs in an optical coupling-branching device.
In the branch waveguide of (1), particularly, in a silicon fine-line waveguide (i.e., a waveguide having a small diameter) of a single mode, in order to reduce a loss due to reflection or scattering in the branching portion, it is required to finely control the shape (curvature, thickness, offset and the like) and a highly precise miniaturization processing technology is required. Further, in the optical coupling-branching device of (2), since interference is used, it is also necessary to perform a highly precise miniaturization process for the shape or the like of the coupling portion with the waveguide and the width and length of the device and it is understood difficult to realize an optical coupling-branching device that can be easily formed. Further, since 3-dimensionally branching light between the optical interconnection layers arranged on the multiple layers in either structure requires a complicated 3-dimensional device structure, it is understood difficult to realize the same.
Thus, conventionally, it is understood difficult to realize the optical coupling-branching device that can be easily formed and distributes light by 3-dimensionally branching the same between different optical interconnection layers and combines the same.